1. Technical Field of the Invention
This invention relates to compositions of matter useful as shields in power cables and to shields and power cables utilizing the composition.
2. Description of the Related Art
Semiconductive shields have been used in power cables as shields for the cable conductor and insulation for many years. A conductor shield is typically extruded over the cable conductor to provide a layer of intermediate conductivity between the conductor and cable insulation in the power cable. A shield is also typically provided over the insulation. Conventional compositions for these shields include a base polymer as the predominant component of the composition compounded with, carbon black to provide conductivity for the composition and various additives including antioxidants, processing aids or lubricants and curing agents.
Examples of polymer compositions used as shields in power cables are found in the disclosures of U.S. Pat. Nos. 4,612,139 and 4,305,846 to Kawasaki et al., U.S. Pat. No. 4,857,232 to Burns, Jr., U.S. Pat. No. 3,849,333 to Lloyd et al., and U.S. Pat. No. 5,889,117 to Flenniken, the disclosures of which are hereby incorporated by reference.
As previously mentioned, one common additive to cable shield compositions is an antioxidant. Conventional antioxidants used in shielding compositions for power cables include stearically hindered phenols and amines such as polymerized 1,2-dihydro-2,2,4-trimethylquinoline, octadecyl, 3,5 diterbutyl-4-hydroxyhydrocinnamate, 4,4'-thio-bis-(3-methyl-6-tert-butylphenol), thiodithylene-'bis-(3,5-ditert-butyl-4-hydroxy) hydrocinnamate, distearyl-thio-dispropionate, and mixtures of these compounds. The antioxidant components are generally included in the shield compositions in amounts significantly less than 1.0% by weight, typically less than 0.7%, by weight, of the shield compositions. The antioxidants function as stabilizers to prevent degradation of the polymer composition over time due to temperature.